An experienced economist and a novice economist are walking down the road. They come across some dog shit lying on the pavement.
The experienced economist says, "If you eat that dog shit, I'll give you $20,000!"
The novice economist runs his optimization program and figures out he's better off eating it, so he does and collects the money.
Continuing along the same road they almost step into another pile of dog shit. The novice economist says, "Now, if you eat this shit I'll give you $20,000."
After evaluating the proposal, the experienced economist eats the shit and collects the money.
They go on. The novice economist wonders, "Listen, we both have the same amount of money we had before, but we both ate shit. I don't see us being better off."
The experienced economist retorts, "Not so! We've created $40,000 of trade!"